1. Field of the Invention
This invention relates generally to a method of upgrading low-rank coal, and more particularly to a method which is suitable for upgrading low-rank coal with a high moisture content, and hence a low heating value, to produce coal with an increased calorific value.
2. Description of the Prior Art
Almost all the coals widely used as fuel are high-rank coals such as bituminous coal. However, low-rank coals such as lignite and subbituminous coal make up about a quarter of the coal on earth but are not sufficiently utilized because they have a large moisture content, or both a large moisture content and a large ash content, or a large ash content, and hence have a low heating value. In order to utilize these low-rank coals effectively as fuel, it is necessary to reduce the moisture and ash content, or either of the moisture content and the ash content, and upgrade them to coals having an increased calorific value.
However, since high-rank coals are primarily used at present, as stated above, suitable upgrading techniques for the effective utilization of low-rank coals yet remain at the research level and are incomplete.
The most popular method of conventional techniques of upgrading low-rank coals is to reduce the large moisture content of low-rank coal by a dryer. A railway transportation test of dried low-rank coals conducted at Grand Forks Energy Research Center, U.S.A., can be cited as typical of such attempts. If the large moisture content of the low-rank coal is merely reduced by a dryer, however, the dehydrated low-rank coal will absorb moisture from the atmosphere or rain water so that the moisture content of the low-rank coal will again increase, and the coal can not be used efficiently. In addition, such a method can not solve the problem that low-rank coals are inherently prone to spontaneous combustion.
Another conventional method is typified by the so-called "Fleissner process" which does not merely dry low-rank coal but modifies its properties per se. In the Fleissner process, low-rank coal is heated at an increased pressure which produces the moisture without evaporating it, and makes the low-rank coal hydrophobic. Accordingly, this process can improve the moisture-proofed property of low-rank coal, and is therefore an effective method of upgrading low-rank coals. In order to upgrade a large mass of low-rank coal by this process, however, an autoclave must be employed and this results in an increase in the cost of equipment. Moreover, since the heat treatment of low-rank coal using an autoclave is carried out in batches, this process is not really suitable for upgrading low-rank coal in practice because the output of low-rank coal is as much as 1,000,000 tons a year.
Still another method of modifying the properties of low-rank coal which does not simply dry it is reported in an article by Obita et al entitled "A Study on the Upgrading of Low-Grade Coals by Heat Treatment", Mitsubishi Juko Giho, Vol. 19, No. 3 (1982-5). In this method, low-rank coal is heated rapidly at normal pressure using a fluidized bed to produce tar on the surface of the low-rank coal, and the tar is kept on the surface of the low-rank coal immediately before it evaporates to vapor. Although this is effective as an method of upgrading low-rank coal, the time during which the properties of the low-rank coals are modified is not sufficiently long because the heat treatment is carried out at high speed, so that the low-rank coal is not rendered sufficiently hydrophobic. Moreover, since a fluidized bed system must always be employed, a critical limitation is imposed upon the particle size of the coal supplied. For these reasons, this method is not really suitable for upgrading low-rank coal on a practical scale.